The study of sleep pattern changes observed in many psychiatric syndromes has already yielded significant findings in the investigation of affective disorders. It is now widely held that all-night electroencephalographic studies supplemented by psychological measurements, telemetric monitoring of activity, and clinical psychopharmacologic techniques could add an important dimension to the differential diagnosis and treatment of mental illnesses. By using a longitudinal approach it is hypothesized (1) that the severity and type of affective disorders can be classified by the investigation of certain sleep parameters; (2) that the efficacy of certain groups of drugs used in the treatment of depression can be tested with a classification schema derived from a constellation of sleep parameters. In particular, it is hypothesized that recurrent depressions consist of two psychobiologically distinct clinical entities: depressive episodes characterized by hypersomnia and decreased psychomotor activity most frequently but by no means exclusively observed in patients suffering from manic-depressive (bipolar) disease, and depressive episodes characterized by hyposomnia and increased psychomotor activity, usually found in patients with the unipolar type of affective disorder. After careful attention has been given to the characterization and diagnosis of the patient's illness, selected patients will be admitted to the clinical research unit at the Western Psychiatric Institute and Clinic for both longitudinal sleep studies and behavioral observations. This intensive approach will be carried out on 75 consecutive admitted patients with depressive syndromes. Changes in sleep pattern, both in relation to the administration of various antidepressant drugs and to the clinical course would be assessed to determine the effectiveness of various psychopharmacologic and other treatment strategies within this new classification schema and provide the predictive criteria for decisions.